nasm/doc/directiv.src

\C{directive} \i{Assembler Directives}

NASM, though it attempts to avoid the bureaucracy of assemblers like
MASM and TASM, is nevertheless forced to support a \e{few}
directives. These are described in this chapter.

NASM's directives come in two types: \I{user-level
directives}\e{user-level} directives and \I{primitive
directives}\e{primitive} directives. Typically, each directive has a
user-level form and a primitive form. In almost all cases, we
recommend that users use the user-level forms of the directives,
which are implemented as macros which call the primitive forms.

Primitive directives are enclosed in square brackets; user-level
directives are not.

In addition to the universal directives described in this chapter,
each object file format can optionally supply extra directives in
order to control particular features of that file format. These
\I{format-specific directives}\e{format-specific} directives are
documented along with the formats that implement them, in \k{outfmt}.


\H{bits} \i\c{BITS}: Target \i{Processor Mode}

The \c{BITS} directive specifies whether NASM should generate code
\I{16-bit mode, versus 32-bit mode}designed to run on a processor
operating in 16-bit mode, 32-bit mode or 64-bit mode. The syntax is
\c{BITS XX}, where XX is 16, 32 or 64.

In most cases, you should not need to use \c{BITS} explicitly. The
\c{aout}, \c{coff}, \c{elf*}, \c{macho}, \c{win32} and \c{win64}
object formats, which are designed for use in 32-bit or 64-bit
operating systems, all cause NASM to select 32-bit or 64-bit mode,
respectively, by default. The \c{obj} object format allows you
to specify each segment you define as either \c{USE16} or \c{USE32},
and NASM will set its operating mode accordingly, so the use of the
\c{BITS} directive is once again unnecessary.

The most likely reason for using the \c{BITS} directive is to write
32-bit or 64-bit code in a flat binary file; this is because the \c{bin}
output format defaults to 16-bit mode in anticipation of it being
used most frequently to write DOS \c{.COM} programs, DOS \c{.SYS}
device drivers and boot loader software.

The \c{BITS} directive can also be used to generate code for a
different mode than the standard one for the output format.

You do \e{not} need to specify \c{BITS 32} merely in order to use
32-bit instructions in a 16-bit DOS program; if you do, the
assembler will generate incorrect code because it will be writing
code targeted at a 32-bit platform, to be run on a 16-bit one.

When NASM is in \c{BITS 16} mode, instructions which use 32-bit
data are prefixed with an 0x66 byte, and those referring to 32-bit
addresses have an 0x67 prefix. In \c{BITS 32} mode, the reverse is
true: 32-bit instructions require no prefixes, whereas instructions
using 16-bit data need an 0x66 and those working on 16-bit addresses
need an 0x67.

When NASM is in \c{BITS 64} mode, most instructions operate the same
as they do for \c{BITS 32} mode. However, there are 8 more general and
SSE registers, and 16-bit addressing is no longer supported.

The default address size is 64 bits; 32-bit addressing can be selected
with the 0x67 prefix.  The default operand size is still 32 bits,
however, and the 0x66 prefix selects 16-bit operand size.  The \c{REX}
prefix is used both to select 64-bit operand size, and to access the
new registers. NASM automatically inserts REX prefixes when
necessary.

When the \c{REX} prefix is used, the processor does not know how to
address the AH, BH, CH or DH (high 8-bit legacy) registers. Instead,
it is possible to access the the low 8-bits of the SP, BP, SI and DI
registers as SPL, BPL, SIL and DIL, respectively; but only when the
REX prefix is used.

The \c{BITS} directive has an exactly equivalent primitive form,
\c{[BITS 16]}, \c{[BITS 32]} and \c{[BITS 64]}. The user-level form is
a macro which has no function other than to call the primitive form.

Note that the space is necessary, e.g. \c{BITS32} will \e{not} work!

\S{USE16 & USE32} \i\c{USE16} & \i\c{USE32}: Aliases for BITS

The `\c{USE16}' and `\c{USE32}' directives can be used in place of
`\c{BITS 16}' and `\c{BITS 32}', for compatibility with other assemblers.


\H{default} \i\c{DEFAULT}: Change the assembler defaults

The \c{DEFAULT} directive changes the assembler defaults.  Normally,
NASM defaults to a mode where the programmer is expected to explicitly
specify most features directly.  However, this is occasionally
obnoxious, as the explicit form is pretty much the only one one wishes
to use.

Currently, \c{DEFAULT} can be used to select \c{RIP}-relative
(\c{REL}) or absolute (\c{ABS}) addressing in 64-bit mode, and the use
of MPX \c{BND} prefixes.

\S{default-rel} \i\c{REL}, \i\c{ABS}: RIP-relative addressing

This sets whether registerless instructions in 64-bit mode are
\c{RIP}-relative or not. By default, they are absolute unless
overridden with the \i\c{REL} specifier (see \k{effaddr}).  However,
if \c{DEFAULT REL} is specified, \c{REL} is default, unless overridden
with the \c{ABS} specifier, \e{except when used with an FS or GS
segment override}.

\c{DEFAULT REL} is disabled with \c{DEFAULT ABS}.

The special handling of \c{FS} and \c{GS} overrides are due to the
fact that these registers are generally used as thread pointers or
other special functions in 64-bit mode, and generating
\c{RIP}-relative addresses is not desired on most platforms.

To specify that \c{FS}- or \c{GS}-relative addresses \e{should} also
be generated as \c{RIP}-relative, specify the \c{ABS} or \c{REL}
keyword with an \c{FS:} or \c{GS:} prefix:

\c DEFAULT REL, FS:ABS, GS:REL

... will make \c{FS}-relative references default to absolute, but all
others, including \c{GS}-relative references, \c{RIP}-relative.

\c{DEFAULT REL} is likely to become the default setting in a future
version of NASM. Specify \c{DEFAULT ABS} explicitly if you need your
code to avoid relative offsets.

\S{default-bnd} \i\c{BND}, \i\c{NOBND}: \c{BND} prefix

If \c{DEFAULT BND} is set, all bnd-prefix available instructions following
this directive are prefixed with bnd. To override it, \c{NOBND} prefix can
be used.

\c  DEFAULT BND
\c      call foo            ; BND will be prefixed
\c      nobnd call foo      ; BND will NOT be prefixed

\c{DEFAULT NOBND} can disable \c{DEFAULT BND} and then \c{BND} prefix will be
added only when explicitly specified in code.

\c{DEFAULT BND} is expected to be the normal configuration for writing
MPX-enabled code.

\H{section} \i\c{SECTION} or \i\c{SEGMENT}: Changing and \i{Defining
Sections}

\I{changing sections}\I{switching between sections}The \c{SECTION}
directive (\c{SEGMENT} is an exactly equivalent synonym) changes
which section of the output file the code you write will be
assembled into. In some object file formats, the number and names of
sections are fixed; in others, the user may make up as many as they
wish. Hence \c{SECTION} may sometimes give an error message, or may
define a new section, if you try to switch to a section that does
not (yet) exist.

The Unix object formats, and the \c{bin} object format (but see
\k{multisec}), all support
the \i{standardized section names} \c{.text}, \c{.data} and \c{.bss}
for the code, data and uninitialized-data sections. The \c{obj}
format, by contrast, does not recognize these section names as being
special, and indeed will strip off the leading period of any section
name that has one.


\S{sectmac} The \i\c{__?SECT?__} Macro

The \c{SECTION} directive is unusual in that its user-level form
functions differently from its primitive form. The primitive form,
\c{[SECTION xyz]}, simply switches the current target section to the
one given. The user-level form, \c{SECTION xyz}, however, first
defines the single-line macro \c{__?SECT?__} to be the primitive
\c{[SECTION]} directive which it is about to issue, and then issues
it. So the user-level directive

\c         SECTION .text

expands to the two lines

\c %define __?SECT?__        [SECTION .text]
\c         [SECTION .text]

Users may find it useful to make use of this in their own macros.
For example, the \c{writefile} macro defined in \k{mlmacgre} can be
usefully rewritten in the following more sophisticated form:

\c %macro  writefile 2+
\c
\c         [section .data]
\c
\c   %%str:        db      %2
\c   %%endstr:
\c
\c         __?SECT?__
\c
\c         mov     dx,%%str
\c         mov     cx,%%endstr-%%str
\c         mov     bx,%1
\c         mov     ah,0x40
\c         int     0x21
\c
\c %endmacro

This form of the macro, once passed a string to output, first
switches temporarily to the data section of the file, using the
primitive form of the \c{SECTION} directive so as not to modify
\c{__?SECT?__}. It then declares its string in the data section, and
then invokes \c{__?SECT?__} to switch back to \e{whichever} section
the user was previously working in. It thus avoids the need, in the
previous version of the macro, to include a \c{JMP} instruction to
jump over the data, and also does not fail if, in a complicated
\c{OBJ} format module, the user could potentially be assembling the
code in any of several separate code sections.


\H{absolute} \i\c{ABSOLUTE}: Defining Absolute Labels

The \c{ABSOLUTE} directive can be thought of as an alternative form
of \c{SECTION}: it causes the subsequent code to be directed at no
physical section, but at the hypothetical section starting at the
given absolute address. The only instructions you can use in this
mode are the \c{RESB} family.

\c{ABSOLUTE} is used as follows:

\c absolute 0x1A
\c
\c     kbuf_chr    resw    1
\c     kbuf_free   resw    1
\c     kbuf        resw    16

This example describes a section of the PC BIOS data area, at
segment address 0x40: the above code defines \c{kbuf_chr} to be
0x1A, \c{kbuf_free} to be 0x1C, and \c{kbuf} to be 0x1E.

The user-level form of \c{ABSOLUTE}, like that of \c{SECTION},
redefines the \i\c{__?SECT?__} macro when it is invoked.

\i\c{STRUC} and \i\c{ENDSTRUC} are defined as macros which use
\c{ABSOLUTE} (and also \c{__?SECT?__}).

\c{ABSOLUTE} doesn't have to take an absolute constant as an
argument: it can take an expression (actually, a \i{critical
expression}: see \k{crit}) and it can be a value in a segment. For
example, a TSR can re-use its setup code as run-time BSS like this:

\c         org     100h               ; it's a .COM program
\c
\c         jmp     setup              ; setup code comes last
\c
\c         ; the resident part of the TSR goes here
\c setup:
\c         ; now write the code that installs the TSR here
\c
\c absolute setup
\c
\c runtimevar1     resw    1
\c runtimevar2     resd    20
\c
\c tsr_end:

This defines some variables `on top of' the setup code, so that
after the setup has finished running, the space it took up can be
re-used as data storage for the running TSR. The symbol `tsr_end'
can be used to calculate the total size of the part of the TSR that
needs to be made resident.


\H{extern} \i\c{EXTERN}: \i{Importing Symbols} from Other Modules

\c{EXTERN} is similar to the MASM directive \c{EXTRN} and the C
keyword \c{extern}: it is used to declare a symbol which is not
defined anywhere in the module being assembled, but is assumed to be
defined in some other module and needs to be referred to by this
one. Not every object-file format can support external variables:
the \c{bin} format cannot.

The \c{EXTERN} directive takes as many arguments as you like. Each
argument is the name of a symbol:

\c extern  _printf
\c extern  _sscanf,_fscanf

Some object-file formats provide extra features to the \c{EXTERN}
directive. In all cases, the extra features are used by suffixing a
colon to the symbol name followed by object-format specific text.
For example, the \c{obj} format allows you to declare that the
default segment base of an external should be the group \c{dgroup}
by means of the directive

\c extern  _variable:wrt dgroup

The primitive form of \c{EXTERN} differs from the user-level form
only in that it can take only one argument at a time: the support
for multiple arguments is implemented at the preprocessor level.

You can declare the same variable as \c{EXTERN} more than once: NASM
will quietly ignore the second and later redeclarations.

If a variable is declared both \c{GLOBAL} and \c{EXTERN}, or if it is
declared as \c{EXTERN} and then defined, it will be treated as
\c{GLOBAL}. If a variable is declared both as \c{COMMON} and
\c{EXTERN}, it will be treated as \c{COMMON}.

Since NASM version 2.15, the \c{EXTERN} keyword (since version 2.15)
does not request import of symbols that are never actually referenced
in the code, as that prevents using common header files, as it might
cause the linker to pull in a bunch of unnecessary modules.  To
unconditionally request import of external symbols, use the
\c{REQUIRED} directive instead (see \k{required}).

If the old behavior is required, rather than changing the source code,
one can override the user macro definition:

\c %ifmacro required	  ; Test for NASM new enough to support REQUIRED
\c   %unimacro extern 1-*
\c   %imacro extern 1+.nolist
\c      required %1
\c   %endmacro
\c %endif

\H{required} \i\c{REQUIRED}: \i{Unconditionally Importing Symbols} from Other Modules

The \c{REQUIRED} keyword is similar to \c{EXTERN} one. The difference
is that the \c{EXTERN} keyword (since version 2.15) does not request
import of symbols that are never actually referenced in the code, as
that prevents using common header files, as it might cause the linker
to pull in a bunch of unnecessary modules.

\H{global} \i\c{GLOBAL}: \i{Exporting Symbols} to Other Modules

\c{GLOBAL} is the other end of \c{EXTERN}: if one module declares a
symbol as \c{EXTERN} and refers to it, then in order to prevent
linker errors, some other module must actually \e{define} the
symbol and declare it as \c{GLOBAL}. Some assemblers use the name
\i\c{PUBLIC} for this purpose.

\c{GLOBAL} uses the same syntax as \c{EXTERN}, except that it must
refer to symbols which \e{are} defined in the same module as the
\c{GLOBAL} directive. For example:

\c global _main
\c _main:
\c         ; some code

\c{GLOBAL}, like \c{EXTERN}, allows object formats to define private
extensions by means of a colon. The ELF object format, for example,
lets you specify whether global data items are functions or data:

\c global  hashlookup:function, hashtable:data

Like \c{EXTERN}, the primitive form of \c{GLOBAL} differs from the
user-level form only in that it can take only one argument at a
time.


\H{common} \i\c{COMMON}: Defining Common Data Areas

The \c{COMMON} directive is used to declare \i\e{common variables}.
A common variable is much like a global variable declared in the
uninitialized data section, so that

\c common  intvar  4

is similar in function to

\c global  intvar
\c section .bss
\c
\c intvar  resd    1

The difference is that if more than one module defines the same
common variable, then at link time those variables will be
\e{merged}, and references to \c{intvar} in all modules will point
at the same piece of memory.

Like \c{GLOBAL} and \c{EXTERN}, \c{COMMON} supports object-format
specific extensions. For example, the \c{obj} format allows common
variables to be NEAR or FAR, and the ELF format allows you to specify
the alignment requirements of a common variable:

\c common  commvar  4:near  ; works in OBJ
\c common  intarray 100:4   ; works in ELF: 4 byte aligned

Once again, like \c{EXTERN} and \c{GLOBAL}, the primitive form of
\c{COMMON} differs from the user-level form only in that it can take
only one argument at a time.

\H{static} \i\c{STATIC}: Local Symbols within Modules

Opposite to \c{EXTERN} and \c{GLOBAL}, \c{STATIC} is local symbol, but
should be named according to the global mangling rules (named by
analogy with the C keyword \c{static} as applied to functions or
global variables).

\c static foo
\c foo:
\c          ; codes

Unlike \c{GLOBAL}, \c{STATIC} does not allow object formats to accept
private extensions mentioned in \k{global}.

\H{mangling} \i\c{(G|L)PREFIX}, \i\c{(G|L)POSTFIX}: Mangling Symbols

\c{PREFIX}, \c{GPREFIX}, \c{LPREFIX}, \c{POSTFIX}, \c{GPOSTFIX}, and
\c{LPOSTFIX} directives can prepend or append a string to a certain
type of symbols, normally to fit specific ABI conventions

\b\c{PREFIX}|\c{GPREFIX}: Prepend the argument to all \c{EXTERN},
\c{COMMON}, \c{STATIC}, and \c{GLOBAL} symbols.

\b\c{LPREFIX}: Prepend the argument to all other symbols
such as local labels and backend defined symbols.

\b\c{POSTFIX}|\c{GPOSTFIX}: Append the argument to all \c{EXTERN},
\c{COMMON}, \c{STATIC}, and \c{GLOBAL} symbols.

\b\c{LPOSTFIX}: Append the argument to all other symbols
such as local labels and backend defined symbols.

These are macros implemented as pragmas, and using \c{%pragma} syntax
can be restricted to specific backends (see \k{pragma}):

\c %pragma macho lprefix L_

Command line options are also available. See also \k{opt-pfix}.

One example which supports many ABIs:

\c ; The most common conventions
\c %pragma output gprefix _
\c %pragma output lprefix L_
\c ; ELF uses a different convention
\c %pragma elf    gprefix			; empty
\c %pragma elf    lprefix .L

Some toolchains is aware of a particular prefix for its own
optimization options, such as dead code elimination. For instance, the
Mach-O binary format has a linker convention that uses a simplistic
naming scheme to chunk up sections into smaller subsections, each of
which may be eliminated. When the \c{subsections_via_symbols}
directive (\k{macho-ssvs}) is declared, each symbol is the start of a
separate block. The subsection is, then, defined to include sections
before the one that starts with a 'L'. \c{LPREFIX} is useful here to
mark all local symbols with the 'L' prefix to be excluded to the meta
section.  It converts local symbols compatible with the particular
toolchain.  Note that local symbols declared with \c{STATIC}
(\k{static}) are excluded from the symbol mangling and also not marked
as global.


\H{CPU} \i\c{CPU}: Defining CPU Dependencies

The \i\c{CPU} directive restricts assembly to those instructions which
are available on the specified CPU. At the moment, it is primarily
used to enforce unavailable \e{encodings} of instructions, such as
5-byte jumps on the 8080.

(If someone would volunteer to work through the database and add
proper annotations to each instruction, this could be greatly
improved. Please contact the developers to volunteer, see \k{contact}.)

Current CPU keywords are:

\b\c{CPU 8086}        - Assemble only 8086 instruction set

\b\c{CPU 186}         - Assemble instructions up to the 80186 instruction set

\b\c{CPU 286}         - Assemble instructions up to the 286 instruction set

\b\c{CPU 386}         - Assemble instructions up to the 386 instruction set

\b\c{CPU 486}         - 486 instruction set

\b\c{CPU 586}         - Pentium instruction set

\b\c{CPU PENTIUM}     - Same as 586

\b\c{CPU 686}         - P6 instruction set

\b\c{CPU PPRO}        - Same as 686

\b\c{CPU P2}          - Same as 686

\b\c{CPU P3}          - Pentium III (Katmai) instruction sets

\b\c{CPU KATMAI}      - Same as P3

\b\c{CPU P4}          - Pentium 4 (Willamette) instruction set

\b\c{CPU WILLAMETTE}  - Same as P4

\b\c{CPU PRESCOTT}    - Prescott instruction set

\b\c{CPU X64}         - x86-64 (x64/AMD64/Intel 64) instruction set

\b\c{CPU IA64}        - IA64 CPU (in x86 mode) instruction set

\b\c{CPU DEFAULT}     - All available instructions

\b\c{CPU ALL}	      - All available instructions \e{and flags}

All options are case insensitive.

In addition, optional flags can be specified to modify the instruction
selections. These can be combined with a CPU declaration or specified
alone. They can be prefixed by \c{+} (add flag, default), \c{-}
(remove flag) or \c{*} (set flag to default); these prefixes are
"sticky", so:

\c      cpu -foo,bar

means remove both the \c{foo} and \c{bar} options.

If prefixed with \c{no}, it inverts the meaning of the flag, but this
is not sticky, so:

\c      cpu nofoo,bar

means remove the \c{foo} flag but add the \c{bar} flag.

Currently available flags are:

\b\c{EVEX} - Enable generation of EVEX (AVX-512) encoded instructions
without an explicit \c{\{evex\}} prefix. Default on.

\b\c\{VEX} - Enable generation of VEX (AVX) or XOP encoded
instructions without an explict \c{\{vex\}} prefix. Default on.

\b\c{LATEVEX} - Enable generation of VEX (AVX) encoding of
instructions where the VEX instructions forms were introduced
\e{after} the corresponding EVEX (AVX-512) instruction forms without
requiring an explicit \c{\{vex\}} prefix. This is implicit if the
\c{EVEX} flag is disabled and the \c{VEX} flag is enabled. Default
off.


\H{dollarhex} \i\c{[DOLLARHEX]}: Enable or disable \c{$} hexadecimal syntax

Using a \c{$} prefix for hexadecimal numbers is deprecated, as it
conflicts with the use of \c{$} for escaping symbols (see \k{numconst}
and \k{syntax}). The \c{[DOLLARHEX]} directive can be used to disable
it completely:

\c     [dollarhex off]

When disabled, symbols beginning with digits can be escaped as well,
e.g. \c{$3} would define a symbol \c{3}.

No "user form" (without the brackets) currently exists.


\H{FLOAT} \i\c{FLOAT}: Handling of \I{floating-point, constants}floating-point constants

By default, floating-point constants are rounded to nearest, and IEEE
denormals are supported.  The following options can be set to alter
this behaviour:

\b\c{FLOAT DAZ}       - Flush denormals to zero

\b\c{FLOAT NODAZ}     - Do not flush denormals to zero (default)

\b\c{FLOAT NEAR}      - Round to nearest (default)

\b\c{FLOAT UP}        - Round up (toward +Infinity)

\b\c{FLOAT DOWN}      - Round down (toward -Infinity)

\b\c{FLOAT ZERO}      - Round toward zero

\b\c{FLOAT DEFAULT}   - Restore default settings

The standard macros \i\c{__?FLOAT_DAZ?__}, \i\c{__?FLOAT_ROUND?__}, and
\i\c{__?FLOAT?__} contain the current state, as long as the programmer
has avoided the use of the bracketed primitive form, (\c{[FLOAT]}).

\c{__?FLOAT?__} contains the full set of floating-point settings; this
value can be saved away and invoked later to restore the setting.


\H{asmdir-warning} \i\c{[WARNING]}: Enable or disable warnings

The \c{[WARNING]} directive can be used to enable or disable classes
of warnings in the same way as the \c{-w} option, see \k{warnings} for
more details about warning classes.

\b \c{[warning +}\e{warning-class}\c{]} enables warnings for
   \e{warning-class}.

\b \c{[warning -}\e{warning-class}\c{]} disables warnings for
   \e{warning-class}.

\b \c{[warning *}\e{warning-class}\c{]} restores \e{warning-class} to
   the original value, either the default value or as specified on the
   command line.

\b \c{[warning push]} saves the current warning state on a stack.

\b \c{[warning pop]} restores the current warning state from the stack.

The \c{[WARNING]} directive also accepts the \c{all}, \c{error} and
\c{error=}\e{warning-class} specifiers, see \k{opt-w}.

No "user form" (without the brackets) currently exists.


\H{LIST} \i\c{[LIST]}: Locally disable list file output

The \c{[LIST]} directive disables or re-enables list file output.

\b \c{[list -]} disables list file output.

\b \c{[list +]} re-enables list file output.

The \c{[LIST]} directive can be overridden with the \c{-LF}
command-line option, see \k{opt-L}.